The art of dispensing liquid onto a spinning surface to provide liquid on the surface is widely used in industry. Reference is made to U.S. Pat. No. 4,267,212 issued May 12, 1981 for a "Spin Coating Process" utilizing a dispensing nozzle to apply photoresist radially over a spinning surface. Also see U.S. Pat. No. 2,487,111, issued Nov. 8, 1949 which describes a spraying process in which the flow rate of a viscous liquid containing coating material is varied. A constant velocity cam is utilized to control the spray at the center of the surface to minimize the tendency to accumulate thick coatings at the central portion.
In the semiconductor processing art in which a photoresist developer liquid, such as 100% Waycoat negative photo resist developer, is to be sprayed on a surface of a wafer device, it is highly desireable that a more uniform spray be applied. In order to do this, it is necessary to repeatedly pass a spray nozzle over the surface many times in order to achieve this desired uniformity of spray by providing equal volume per unit area of the liquid.
In one prior art device, the nozzle is reciprocated over the surface by means of a double acting air cylinder which drives the nozzle in a reciprocating diametric path over the wafer surface. The spray volume per unit area of this photoresist developer applied by such mechanism tends to be uneven primarily due to the constant velocity motion characteristics of an air cylinder nozzle moving the nozzle over the surface. Since the nozzle traverses the wafer from a maximum circumference area through a minimum circumference area and again to a maximum circumference area with a constant volume delivery, the applied spray of the developer tends to be heavier towards the center of the wafer and thinner at the outside edges. Moreover, since an air cylinder starts and stops abruptly at its extreme positions (causing sudden high acceleration and deceleration), mechanical vibrations are introduced into the dispensing nozzle resulting in an uneven distribution of the liquid over the wafer surface. This poor distribution is undesireable since non-uniform distribution of the developer tends to cause over-etching during the developing process on the outside of the wafer and, moreover, under-etching at the inside areas. Such undesirable effects obviously reduce the yield of a product line.
There is a need in the art for a spraying apparatus that provides consistently uniform quantities of the liquid repeatedly and reliably over unit areas.